Known in the past has been an internal combustion engine which is provided with an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and is configured to control the amount of fuel which is fed to the internal combustion engine based on the output of this air-fuel ratio sensor,
The air-fuel ratio sensor used in such an internal combustion engine gradually deteriorates along with use. As such deterioration, for example, deterioration in response of the air-fuel ratio sensor may be mentioned. Deterioration in response of the air-fuel ratio sensor occurs, for example, due to the air-holes, which are provided in the sensor cover for preventing the sensor element from being covered with water, being partially clogged by fine particles. If the air holes are partially clogged in this way, the exchange of gas between the inside and outside of the sensor cover becomes slow and, as a result, the response of the air-fuel ratio sensor is dulled. If such deterioration in response of the air-fuel ratio sensor occurs, the various types of control, which the control system of the internal combustion engine performs, is obstructed.
Therefore, an abnormality diagnosis system which diagnoses the abnormality of deterioration in response of the air-fuel ratio sensor has been proposed (for example, PTLs 1 to 4). As such an abnormality diagnosis system, for example, a system, which diagnoses abnormality of the air-fuel ratio sensor based on the change of output value of the air-fuel ratio sensor which change accompanies the start of fuel cut control for stopping the feed of fuel to a combustion chamber, has been proposed (for example, PTL 1).
In particular, in the abnormality diagnosis system described in PTL 1, the response time from the start of fuel cut control to when the output voltage of the air-fuel ratio sensor falls to a prescribed value, is measured and the response time, is corrected based on the relationship between the response time and the average amount of intake air. Further, when the thus corrected response time is a predetermined reference response time or more, it is judged that the air-fuel ratio sensor has become abnormal. According to PTL 1, due to this abnormality diagnosis system, even if the air-fuel ratio sensor is a downstream side air-fuel ratio sensor provided at a downstream side, in the exhaust flow direction, of the exhaust purification catalyst, it is considered possible to diagnose abnormality of the downstream side air-fuel ratio sensor without being affected by deterioration of the exhaust purification catalyst.